Locks have been in use for thousands of years, securing property from intrusion, tampering or theft. Some of the earliest locks used a pin-lock mechanism which included a wooden lock and key. The lock was strung on a rope hanging out of a hole in a door. A cylinder of wood with a hole drilled through its axis served as the key. The cylinder may be inserted into the hole to push a bolt in the door a predetermined distance to unlock the door. To lock the door, the rope may be pulled to extract the cylinder while simultaneously pulling the bolt closed.
Generally, barrel locks provide effective security against tampering or malicious unlocking. However, in certain instances, where a force is applied to the lock that is greater than the force provided by a compression spring, the lock may pop open. Thus, barrel locks may be vulnerable to a malicious pop open condition.